Non-Foster antenna matching networks utilize non-Foster circuits (NFCs) which can, in theory, overcome limitations of traditional passive circuits by using active non-Foster circuits to synthesize negative capacitors and negative inductors. Electrically small antennas tend to have capacitive reactance and using a negative capacitive reactance in an Electrically Small Antenna (ESA) matching network can be used to offset largely the capacitance of the antenna and thereby produce a good match to the antenna over a very wide range of frequencies.
Traditional antenna matching circuits (which use passive elements, capacitors and possibly inductors), are frequency dependent, so that an ideal match only occurs at one frequency (or a small set of frequencies for complex matching networks). The match is typically satisfactory at frequencies near the ideal match frequency, but as the desired frequency gets further and further away from the ideal frequency, the match worsens and antenna performance suffers. In the prior art, this problem of match degradation has been dealt with by tuning the antenna matching circuits as the receiver and/or transmitter changes frequency.
This has worked well in the context of narrow band receivers and transmitters which only needed to have a relatively small bandwidth to accommodate whatever demodulation or modulation techniques they employed.
However, there are applications for wide bandwidth receivers and transmitters that need to be responsive or operate over wide bandwidths.
Non-foster antenna matching circuits for ESAs have been proposed in the prior art by J. G. Linvill and Stephen E. Sussman-Fort, et. al, to improve the signal to noise ratio (SNR) of antenna systems. See, J. G. Linvill, “Transistor Negative Impedance Converters,” Proc. IRE, vol. 41, June 1953 and Stephen E. Sussman-Fort and Ronald M. Rudish, “Non-Foster Impedance Matching of Electrically Small Antennas”, IEEE Transactions on Antennas and Propagation, Vol. 57, August 2009. They teach a wide band matching method and a corresponding matching circuit, such as a negative impedance inverter (NII) or a negative impedance converter (NIC). See also R. R. Hoskins, “Stability of negative impedance converters,” Electronics Letters, vol. 2, no. 9, September 1966. An ESA is generally defined as an antenna whose maximum dimension (of an active element) is no more than wavelengths at the highest frequencies at which the antenna is expected to operate. So, for a dipole with a length of λ/2π, a loop with a diameter of λ/2π, or a patch with a diagonal dimension of λ/2π would be considered electrically small.
A non-foster network has been proposed to achieve wideband matching between a receiver and an ESA. The non-foster network overcomes the narrow bandwidth and poor gain associated with passive matching circuits that are severely limited by gain-bandwidth theory. FIG. 1 shows a non-foster matching configurations for the electrically small antenna published by Stephen E. Sussman-Fort.
The non-foster matching network of FIG. 1 utilizes series/shunt negative capacitor or inductor to implement the wide band matching. FIG. 2 gives one example for realizing the series negative capacitor to achieve such a matching.
Normally, negative capacitance is realized through positive feedback in a NFC and, therefore, oscillates when connected to an improper load impedance. On the other hand, NFCs achieve the best gain performance when on the verge of oscillation. Therefore, tight tolerances are typically required to achieve non-Foster antenna matching circuits.
While there are examples of the use of NFCs in rather precise laboratory settings, no NFCs are found in use in commercial products today because the required circuit parameters typically change as a function of environmental factors (where the antenna is physically located, for example), environmental effects (such as temperature), circuit fabrication tolerances and circuit aging. To date, no solutions to these problems have appeared. There is a need for more robust NFCs that remain stable in a high-performance matching state despite environmental changes and fabrication tolerances, etc.
While a robust NFC is especially important for ESAs, a robust NFC is also quite useful in many other applications, including, for example, in antenna matching circuits which match antennas which are larger than a typical ESA.